1. Field of the Invention
The present invention relates to a method of a simultaneous preparation of hexafluoropropylene (hereinafter, referred to as xe2x80x98HFPxe2x80x99) and octafluorocyclobutane (hereinafter, referred to as xe2x80x98RC318xe2x80x99).
2. Description of the Background Art
In general, HFP is a monomer used as a raw material for a fluoride group resin copolymer, and a principal raw material for a fluoride resin together with tetrafluoroethylene (hereinafter, referred to as xe2x80x98TFExe2x80x99).
As preparation methods of HFP, a simultaneous preparation of TFE and HFP through a thermal decomposition of difluorochloromethane (hereinafter, referred to as xe2x80x98R22xe2x80x99) (EP No. 0 287 219 and U.S. Pat. No. 4,849,554), a thermal decomposition of TFE and RC318 (U.S. Pat. No. 3,446,858), a thermal decomposition of polytetrafluoroethylene (U.S. Pat. No. 2,759,983) and a thermal decomposition of trifluoromethane (hereinafter, referred to as xe2x80x98R23xe2x80x99) (U.S. Pat. No. 3,009,966) have been reported.
In addition, EP No. 0 287 219 and U.S. Pat. No. 4,849,554 disclose a simultaneous preparation of TFE and HFP through a thermal decomposition of R22 at 750-980xc2x0 C. for 1-50 milliseconds of a contact time as shown in the below reaction scheme 1.
Reaction Scheme 1
CF2HClxe2x86x92CF2xe2x95x90CF2+CF2xe2x95x90CFCF3
In this method, if the reaction temperature is below 880xc2x0 C., a conversion of R22 is 40% and a selectivity of HFP is below 9%. If the reaction temperature is 930xc2x0 C., a conversion of R22 is 50% and a selectivity of HFP is 20%. However, the above preparation method through a thermal decomposition of R22 is not suitable for preparing HFP. It is because there is a difficulty in separation and purification of HFP produced due to its azeotrope formation with R22 which is a starting material, and the selectivity of HFP is low while the selectivity of TFE is high.
In U.S. Pat. No. 2,759,983, Waddell et at. discloses a process for preparing 58% of HFP, 37% of TFE and 2% of RC318 through a thermal decomposition of polytetrafluoroethylene at a reaction temperature of 750-960xc2x0 C. and a contact time of less than 5 seconds.
RC318 has been used as a propellant, a refrigerant and a cleaner for a semiconductor. As it has been verified with an excellent efficacy as a refrigerant for a turbo compressor of an air-conditioner, it is expected to be a substantially increased demand for it in a freezer market. Recently, Dupont Co. has announced the use of RC318 as a cleaner gas for a semiconductor chemical vapor deposition (CVD) chamber that can replace hexafluoroethane used as a conventional semiconductor cleaner. Due to an influence of hexafluoroethane on global warming, it is anticipated to be highly increased demand for RC318.
As preparation methods of RC318, there have been reported a thermal decomposition of R22 (U.S. Pat. No. 2,384,821), a dimerization of TFE (U.S. Pat. No. 2,404,374, EP No. 0 451 793 and Japanese Patent No. 57-59,822 and Simons electrochemical fluorination using ethylene and TFE as starting materials (European Patent No. 0 445 399) and a thermal decomposition of PTFE (U.S. Pat. Nos. 2,394,581 and 2,759,983) and the like.
Harmon et al. have reported that at least 80% of RC318 is produced when TFE is sufficiently reacted under a super-atmospheric pressure at a temperature of 125-500xc2x0 C. (U.S. Pat. No. 2,404,374). Atkinson and Trenwith have reported that they have conducted a kinetic study related to a thermal decomposition reaction of TFE and a thermal decomposition reaction of RC318, and that when TFE is dimerized at a temperature lower than 600xc2x0 C., RC318 is mainly produced (J. Chem. Soc., 2082 (1953)). The preparation method of RC318 through the dimerization of TFE at 500xc2x0 C. is as shown in the following reaction scheme 2.
Reaction Scheme 2
2 CF2xe2x95x90CF2xe2x86x92C4F8
The most general preparation method of RC318 is to dimerize TFE in a tubing reactor. However, because the dimerization of TFE is a severe exothermic reaction, this method has a difficulty in controlling the reaction temperature. In this method, TFE is decomposed into carbon and CF4 at a high temperature, which are accumulated at an outlet of a reactor together with a solid polymer produced and cause a pressure increase in a reaction system. When carbon is accumulated inside the reactor, a reaction pressure is increased, a contact time of TFE is lengthened, and accordingly, the amount of heat generation is increased, so that the reaction proceeds continuously. As a result, the reaction temperature instantaneously rises up above 1000xc2x0 C., reaching to a situation that the reaction to be stopped. Therefore, the method for preparing RC318 by performing a dimerization of TFE in the tubing reactor is not suitable for being commercialized due to the difficulty in controlling the reaction temperature increase resulting from the severe exothermic reaction.
Hukuseishiro et al. have reported a method for preparing RC318 using TFE and ammonia as starting materials (Japanese Patent No. 57-59,822). In this method, TFE is supplied by bubbling into ammonia, or TFE and ammonia are supplied with respective quantitative pumps into a reactor. The reaction is then carried out at 570-700xc2x0 C. under a reaction pressure of 0-5 atm with a contact time of 0.3-5 seconds. It has been reported that the conversion is 50-70% and the selectivity of RC318 is 85-86%. However, in this process, if the reaction rate is high, a polymer is produced so as to clog the reaction tube, while if the reaction speed is low, a reaction rate is low. Therefore its industrial utility is low.
A method for preparing RC318 through a thermal decomposition of PTFE has been presented by Benning et al. in 1946 (U.S. Pat. No. 2,394,581). It has been reported that when a thermal decomposition of PTFE is carried out at 575xc2x0 C., 43% of RC318 is produced.
William Ves et al. disclose a method for preparing RC318 from TFE, ethylene and fluoride (European Patent No. 0 455 399). In this method, ethylene and TFE are reacted in a molar ratio of 10:1 to prepare cyclo-C4F4H4 and then Simons electrochemical fluorination is carried out at 40-50xc2x0 C. to give RC318. This method is as shown in the following reaction schemes 3 and 4, However, this method is unadvisable because the method itself is complicated, costs for devices are high, and corrosion of devices is severe.
Reaction Scheme 3
CH2xe2x95x90CH2+CF2xe2x95x90CF2xe2x86x92cyclo-C4F4H4
Reaction Scheme 3
4 HF+cyclo-C4F4H4xe2x86x92C4F8+4 H2
Therefore, an object of the present invention is to provide a simultaneous and selective preparation method of hexafluoropropylene and octafluorocyclobutane by stably carrying out a dimerization of TFE, by which solve the problems in the prior art including the difficulty in temperature control in case of carrying out a dimerization of TFE which is a severe exothermic reaction in a tubing reactor and the difficulty in operating a reactor caused by the problems in controlling reaction temperature, and the formation of carbon and solid polymer.
The above and other objects of the present invention, as embodied and broadly described herein, can be achieved by carrying out a dimerization of TFE in the presence of steam in a fluidized bed reactor system equipped with a nozzle for supplying steam.